It just depends on the machine, some run good down to 70-75 lbs, I've got one the owner and I can't believe is still running at a forever rate of 65 lbs, never fails to start, but it's a miracle old machine. When they get this way, they have a lot of RPM's and not much power.

Honestly, 100.
You might can do with 90, but like the man said the lower it goes the more you end up with something that kinda runs.
Forget what is new, I think 120?
Not sure but 100 is acceptable, I just don't recommend much lower, 100 is an easy to remember, good as an all-around kind of a figure.